The present disclosure relates to a power amplifier circuit. In general, bipolar transistors are used as amplifiers in power amplifier circuits mounted on mobile communication apparatuses such as cellular phones. Bipolar transistors have thermal positive feedback characteristics in which as the temperature of a transistor element increases, the amount of a collector current increases, which causes a further increase in the temperature and an accompanying increase in the amount of the collector current. Thus, for example, a configuration is known in which a resistor element (hereinafter, may also be referred to as a “ballast resistor”) is inserted between the base of a bipolar transistor and a base bias voltage supply terminal in order to suppress an increase in the amount of a collector current caused by a temperature increase.
With such a configuration including a ballast resistor, when the amount of a current flowing from the base bias voltage supply terminal to the base of the amplifier increases as the power level of an input signal increases, a voltage drop in the ballast resistor increases. Therefore, the base voltage of the amplifier drops. Accordingly, a phenomenon may occur in which the amplitude of the base current is not accompanied by the amplitude of a collector current and a power gain thus decreases (hereinafter, may also be referred to as a “gain compression”).
A power amplifier including an impedance circuit that blocks DC components but transmits AC components to reduce the gain compression is disclosed, for example, Japanese Unexamined Patent Application Publication No. 2005-6212. With this configuration, even if the amount of a current flowing from the base bias voltage supply terminal to the base of the bipolar transistor increases, part of the current flows to the impedance circuit, and therefore, a voltage drop in the ballast resistor may be reduced. Furthermore, by changing the impedance of the impedance circuit using an external control signal, the ratio of the amount of a current flowing in the impedance circuit and the amount of a current flowing in the ballast resistor may be adjusted.
With the configuration disclosed in Japanese Unexamined Patent Application Publication No. 2005-6212, when signal amplitude is transmitted to the emitter of the bipolar transistor included in the impedance circuit at the time when a large signal is input, the signal amplitude may be propagated to the base of the bipolar transistor via a parasitic capacitance between the base and emitter of the bipolar transistor. In this configuration, a control voltage is supplied to the base of the bipolar transistor via the resistor element, and the base voltage may thus vibrate in accordance with vibrations of the emitter voltage. Therefore, a sufficient current does not flow between the collector and emitter of the bipolar transistor, and a function as a bypass circuit cannot be fully achieved.